move property definition/access to Message

MessageProperties is currently defined in Frame::Content, which makes it
cumbersome to use when working directly with a Message itself. I attempted
to make this easier by using typedefs, but it's become pretty obvious that
it belongs in the Message class itself.
add accessors for custom headers

Moved from direct access to a Frame::TableField to accesors to that
data. More readable/useable api for this data.